This invention relates to sealing means employed in a rotary machine, such as a turbine or compressor, and in particular to seals used to prevent a process gas from escaping about the shaft of the rotary machine.
In order to accommodate a rotating shaft extending through a wall, as for example in a compressor or turbine, it is necessary to provide clearance between the shaft and the wall. The process gas, or high pressure gas, on one side of the wall tends to flow along the shaft of the rotary machine through the clearance to the atmosphere or other low pressure region on the other side of the wall. Therefore the shaft of the rotary machine must be equipped with sealing means for preventing gas flow or leakage through this clearance between the shaft and the wall. Likewise, specific applications often require that the process gas which contacts one part of the rotary shaft be prevented from coming in contact with or intermixing with fluids at another portion of the shaft.
Labyrinth and carbon ring type seals are commonly interposed between the atmosphere and the process gas area to restrict gas leakage to a minimum. Carbon ring type seals are normally effective in applications where the shaft speeds are relatively slow. In high speeds and high pressure applications the labyrinth type seal is more practical since there is no contact between the seal and the shaft. Very often it is desirable to completely isolate the process gas in the compressor when labyrinth type or carbon type seals are used. Accordingly, a small amount of buffer fluid may be injected between the process fluid and the seal to establish a buffer or barrier. The buffer fluid is generally injected into a region at a pressure slightly above the pressure of the process fluid. From the injection point, the high pressure buffer fluid generally flows in opposite directions along the shaft, respectively toward a low pressure region or atmosphere and toward the working or process gas region of the compressor. Since very low differential pressures are involved, between the process fluid and buffer fluid, the consumption of the buffer fluid is small. The process fluid is prevented or restricted from escaping to the atmosphere by the labyrinth seal, which provides an effective seal at high pressure conditions.
A labyrinth seal is less effective at low pressure differentials, such as occur when the shaft is not rotating or is rotating at relatively low speeds. Under these conditions a carbon ring seal is more desirable. Labyrinth seals become effective at high speed and pressure differentials. However, carbon seals are less desirable at high speeds since they contact the shaft and are subjected to excessive wear.
In applications where both high speed and low speed and high pressures and low pressures are encountered such as lubrication oil cavities of turbochargers, it is necessary to be able to effectively seal at both sets of conditions. The combination of a labyrinth type seal and a carbon ring seal permits effective use of a single seal system in machines that have varying shaft speeds and operating pressures.
Prior art devices use a combination of two sealing devices to restrict the flow of gas along the shaft. The first seal in the sealing device is preferably of the labyrinth type and is mounted encompass the shaft. The second seal may be a carbon or graphite ring seal which operates when the shaft is at rest to restrict the gas flow along the shaft. When the shaft is rotated the carbon ring seal is moved axially by a centrifugal mechanism to a position where it is ineffective in sealing the operting gas. In this type of application a complicated centrifugally operable device is required to move the carbon seal out of sealing engagement with the shaft. No such device is required in the invention disclosed herein. Normally available buffer gas is utilized to move the carbon seals without requiring the use of additional moving parts.